That thymosin, an extract of fetal calf thymus, improves in vitro and in vivo cellular immune response in children with thymic deficiences forms a rationale for determination of the effects of thymosin in vitro on T cell levels in patients with solid malignancies. Tumor-bearing patients with solid malignancies, patients receiving radiation therapy to head and neck, mediastinal, and pelvic portals, patients clinically cured of solid malignancies, and normal controls all show an inverse relationship between initial T cell levels and the in vitro response to thymosin: With progressively lower initial T cell levels the increase after incubation with thymosin increased progressively. As a group, the greatest increase occurred in patients receiving radiation therapy for mediastinal malignancies. The data constitute a basis for a determination of the effects of thymosin on cell-mediated responses and clinical course in patients with solid malignancies. The demonstrations that murine thymocytes exposed to thymosin have increases in cyclic GMP levels form the basis for current attempts to correlate in vitro response to thymosin and cGMP levels in human lymphocytes. BIBLIOGRAPHIC REFERENCES: Kenady, D.E., Chretien, P.B., Potvin, C., and Simon, R.M.: Thymosin reconstitution of T cell deficits in vitro in cancer patients. Cancer 39:575-580, 1977. Kenady, D.E., Chretien, P.B., Potvin, C., and Simon, R.M.: Effect of thymosin in vitro on T cell levels during radiation therapy: Correlations with radiation portal and initial T cell levels. Cancer 39:642-652, 1977.